Having demonstrated by indirect methods that the immature gastrointestinal tract of newborn infants is more permeable to cow milk antigens than the mature intestine of older infants, we extend these studies in man and in an experimental animal model in order to determine by direct radioimmunoassay techniques the extent of antigen transport from the gastrointestinal lumen into the serum. The degree of antigen uptake will be compared with prematurity of infants and with maturational criteria (mitotic index, morphologic features and DNA content) in the intestine of experimental animals. We also plan to characterize the enteromammary secretory immune system with respect to function in control of antigen uptake from the small intestine. Since the newborn infant is devoid of an active secretory system (SIgA system) during infancy, we plan to determine if human colostrum/milk can adequately function as a passive local protective process to prevent penetration of antigens from the intestinal lumen. The exact mechanism of colostral antibody function on the intestinal surface will be investigated and the distribution of colostral antibodies on the gut surface will be determined using a combination of morphologic, physiologic and immunologic techniques. Finally, the "trophic" effect of human colostrum in limited absorption studies on premature infants will be examined using organ culture techniques as well as in vivo studies, we will also examine the effect of colostrum on the fetal/newborn rabbit intestine by using quantitate techniques of maturation of epithelial cells using villus/crypt enzyme markers and cell turnover techniques and morphologic assessment of epithelial cell states. These studies may help to revise our concepts of feeding practices in premature infants. If positive, the studies will support the use of colostrum in premature infants as a protective and "trophic" substance as well as a nutrient.